A man with a sword that knows how to use one is much more dangerous than a man with a gun that doesn't. Synthaxx is right - the problem with dogs are the owners. An owner looking for a dog to protect himself will generally go for dogs of certain breeds over others. With the right amount of intent you can make nearly any animal vicious, but some quite clearly are already genetically pre-disposed to be better at it than others, given the right (read: wrong) training.